In a U.S. Pat. No. 4,158,285 entitled "Interactive Wristwatch Calculator" filed Feb. 9, 1976 by Edward A. Heinsen, et al., a watch/calculator is disclosed which can perform, inter alia, arithmetic operations with real time quantities to produce a real time answer. This is accomplished by providing means for transferring time data from clock circuitry in the watch/calculator to calculator circuitry and back again. The clock circuitry includes a register for storing data representing time, and this time data is periodically updated or incremented by an incrementer circuit, as is more fully described in the referenced patent application. The calculator circuitry includes registers for storing and arithmetically manipulating data, and one of these registers is connected to the clock register by a bidirectional data bus.
When an arithmetic operation is to be performed on time data, this data must be transferred to one of the calculator registers where the operation is to be performed. However, during the transfer and arithmetic operations process, an update or increment signal could be generated by an incrementer circuit in the clock circuitry. Ordinarily, this increment signal would increment the time data in the clock register by one second, since the update signals occur once a second. However, if the time data is in the calculator circuitry when the increment signal occurs, the update signal will be lost, introducing a one-second error. Repeated operations by the calculator circuitry on time data could result in an undesirable cumulative time error.